Weird artifacts in shadow when rendering transparent objects
Denoisers use an algorithm to predict and remove noise (the fireflies). However, when your image gets too noisy, the algorithm will be unable to predict what is supposed to be where (remember it's looking at the pixels and running some sort of math to try and guess what you don't want in the scene, and how to use the surrounding pixels to fix it).

In this case, you're going to need to increase your sample count.